In a polymerization process for preparing polyolefins using a vapor-phase fluidized-bed, the fluidized-bed reactor is previously fed with resin powder called seed polymer and it is then fluidized. The polymerization reaction is carried out by continuously feeding mixed gas materials, catalysts and alkylaluminum compounds as catalyst promoters. Simultaneously with the above operation, impurities (oxygen, moisture or the like) in the gas are removed. The polymer particles which grew up during a predetermined residence time are taken out. When the above-mentioned seed polymers are not used, the fed catalyst cannot be well dispersed to form granular resins; therefore, the fluidized-bed is not formed. Accordingly, in a fluidized-bed polymerization reactor, seed polymers have inevitably been used in the start of operation.
In the production of polyolefins using a vapor-phase fluidized-bed, it is most important that the fed catalysts be dispersed evenly in a reactor and the fluidized gas is also distributed uniformly in the reactor, thereby effectively removing the heat of reaction. That is, when the concentration of catalyst is extremely high locally in a reactor or the cooling effect is insufficient because of the lack of the dispersion of gases, molten resins are formed into lumps which hinder the fluidization, and the temperature distribution becomes more uneven resulting in the formation of much more molten resins. Owing to this vicious circle, the produced resin cannot be withdrawn from the vessel and the reaction must be stopped.
Concerning the above problems, the latter uniform distribution of fluidized gas can be solved rather easily by examining the relationship between particle size, its distribution, the bulk density of resins, and fluidizing gas velocity, with taking the structure of the vessel into consideration. However, regarding the former problem of the dispersion of catalyst, the uniform dispersion has been hardly realized, because the fine powder of catalyst clings to the wall of the vessel due to static electricity generated by the flow of catalyst and resin powder, thereby locally increasing the catalyst concentration. In many cases, this phenomenon is caused to occur markedly in about half a day after the beginning of reaction, and the temperature is raised only in the wall portion wherein the melting of resin is caused to occur.
It is often experienced that resin powder is statically charged when it is fluidized. For example, it is known that when resin powder is transferred through a pipe, it clings thinly to the inside surface of the pipe. In the production of polyolefins with a fluidized-bed, such a phenomenon was also experienced heretofore. As the countermeasures for this, U.S. Pat. No. 4,855,370 discloses that a reactor is fed with moisture-containing gas; Japanese Laid-Open Publication No. 56-4608 discloses a method to allow liquid hydrocarbons to coexist; furthermore, U.S. Pat. No. 4,532,311 discloses the addition of chromium compounds; and Japanese Laid-Open Publication No. 1-230607 discloses the addition an alcohol or ketone into a reactor. In all of these methods, specific substances are supplied into a reactor during polymerization reaction; therefore, particular apparatus must be installed when practiced, and the operation is naturally complicated. Accordingly, a much simpler measure to eliminate effectively the above disadvantages is eagerly required.
The object of the present invention is to provide a method for producing polyolefin particles, which method is free from the above disadvantages in a polymerization reaction with a vapor-phase fluidized-bed, and easily prevents the formation of molten resin due to the occurrence of static electricity without any additional installation to a reaction system.